1. Technical Field
This invention relates to the technology of catalytically converting emissions of a compressed natural gas (CNG) fueled engine, and more particularly to the technology for converting such exhaust gases when the combustion process for such engine is operated at or slightly lean of stoichiometry.
2. Discussion of the Prior Art
Natural gas (essentially 85% methane) is an attractive source of fuel for vehicles because it provides for a lower fuel cost, longer engine life, lower maintenance, and reduced oil consumption. Development of catalysts for high efficiency removal of saturated hydrocarbons, which includes methane, by oxidation within an exhaust stream is of strategic importance; it may be crucial in view of the emission control requirements promulgated by the U.S. Government. In the past, oxidation of methane has received little attention in automotive catalysis. Extreme difficulty of removal of methane is experienced because a C--H bond must be ruptured. In the oxidation of higher alkanes, oxidation is easily achieved by cleavage of C--C bonds. Since the C--H bond is stronger, methane is more difficult to oxidize.
In copending U.S. Ser. No. 07/772,318, filed Oct. 7, 1992 authored by some of the authors of this invention and commonly assigned to the assignee herein, a catalyst is disclosed which enhances the three-way conversion capability of a modified Pd/Al.sub.2 O.sub.3 catalyst in treating the exhaust gas of a compressed natural gas-fueled engine, provided the engine is limited to being operated slightly rich of stoichiometry, i.e., 1.1-1.2 R (R being the ratio of reducing components to oxidizing components in the exhaust gas). Although this is a significant achievement over the prior art, fuel-rich operation affects the fuel economy of operating the CNG fueled engine and therefore can be undesirable. At stoichiometry or below, the conversion capability of such catalyst drops dramatically.
Zeolite catalysts have been found useful for converting nitric oxide contained in the exhaust of a conventional gasoline-fueled engine, particularly when the combustion process is lean (possessing a high excess oxygen content in the exhaust gas). One of the earliest applications of high silica zeolites to the purification of a conventional gasoline-fueled engine exhaust is disclosed in U.S. Pat. No. 4,297,328, wherein a copper-exchanged zeolite is deployed. Copper is most effective as the ion exchange metal because it is more active at lower temperatures than other metals known to date. Such catalyst performed only in an oxidizing environment.
U.S. Ser. No. 07/753,780, filed Sep. 3, 1991, authored by some of the inventors herein and commonly assigned to assignee of this invention, disclosed a modification to such catalyst to prevent it from degrading at high temperatures, usually found in automotive exhaust systems, and to enhance the catalytic activity of copper-exchanged zeolites. Such prior art knowledge has not extended to the use of zeolites for conversion of exhaust gas of a compressed natural gas engine nor to the use of such catalyst when the engine is calibrated at stoichiometry or slightly lean thereof. This is an important differentiation because the exhaust gas from a conventional gasoline-fueled engine will contain considerably more hydrocarbon (HC), nitric oxide (NO), carbon monoxide (CO) concentration compared to the exhaust gas from a CNG-fueled engine (see Table I). Therefore, the ability of a zeolite catalyst to provide any successful degree of conversion for a CNG-fueled engine has not been envisioned and likely would not operate properly based upon data to date.